Coastal vegetation is efficient in damping incident waves even in storm events, thus providing valuable protections to coastal communities. However, large uncertainties lie in determining vegetation drag coefficients (C D), which are directly related to the wave damping capacity of a certain vegetated area. One major uncertainty is related to the different methods used in deriving C D. Currently, two methods are available, i.e. the conventional calibration approach and the new direct measurement approach. Comparative studies of these two methods are lacking to reveal their respective strengths and reduce the uncertainty. Additional uncertainty stems from the dependence of C D on flow conditions (i.e. wave-only or wave-current) and indicative parameters, i.e. Reynolds number (Re) and Keulegan-Carpenter number (KC). Recent studies have obtained C D-Re relations for combined wave-current flows, whereas C D-KC relations in such flow condition remain unexplored. Thus, this study conducts a thorough comparison between two existing methods and explores the C D-KC relations in combined wave-current flows. By a unique revisiting procedure, we show that C D derived by the direct measurement approach have a better overall performance in reproducing both acting force and the resulting wave dissipation. Therefore, a generic C D-KC relation for both wave-only and wave-current flows is proposed using direct measurement approach. Finally, a detailed comparison of these two approaches are given. The comprehensive method comparison and the obtained new C D-KC relation may lead to improved understanding and modelling of wave-vegetation interaction.
The reliability of machine tool is getting more and more attention. Reasonable reliability allocation is beneficial to improve the inherent reliability of machine tool. However, the existing reliability allocation methods for machine tool have some limitations. For example, static part is selected as the reliability allocation object of machine tool, which cannot reflect the characteristics "function realized by motion"; factors affecting reliability allocation are not considered comprehensively, weights of experts are treated as the same, and the allocation results are not optimized or the impact of time on enterprise is neglected in the optimization. To solve these problems, a new multiobjective optimization reliability allocation method for machine tool is proposed in this paper. Firstly, the latest achievements about meta-action are given, and meta-action is set as the reliability allocation object. Secondly, more reasonable and comprehensive factors affecting reliability allocation are extracted. Thirdly, expert weight coefficient is brought to reduce the subjective impact of expert scoring. Fourthly, time factor is brought to make the optimized allocation results more reasonable and accurate. Finally, a numerical control (NC) machine tool made in China is taken as an example, with a comparison on the reliability allocation results of current methods and proposed method. The results verify the applicability, rationality, and accuracy of the proposed method, which lays a foundation for the subsequent study on the quality characteristics of machine tool based on meta-action.
Dynamic characteristics have an important impact on the reliability of computer numerical control (CNC) machine tool. As the basis of dynamic characteristics analysis, the modal analysis should be conducted. However, the current modal analysis methods have some shortcomings. For example, the selected research object cannot reflect the dynamic characteristics of the CNC machine tool, the selection of research object lacks a scientific basis, and the influence of coupling factors is also often ignored. To address these problems, the concepts of meta-action failure mode and the latest research results about meta-action were given herein. Meta-action unit is the smallest structural unit to ensure the normal operation of the machine tool, and it can fully reflect the dynamic characteristics of a machine tool, so it is taken as the research object. In order to reduce the blindness of selecting the analytic object when implementing the reliability improvement measures, we put forward the concept of a key meta-action unit and gave the corresponding extraction method. Meanwhile, electromechanical coupling was considered when built the dynamic model of a key meta-action unit. A CNC machine tool made in China was taken as an example, and a key meta-action unit was extracted and analyzed, then the corresponding reliability improvement measures were also given. The results verify the applicability and effectiveness of this method. The proposed method can solve the shortcomings of the current methods, which lays a foundation for further research of mechanical properties and reliability based on machine tool meta-action unit. INDEX TERMS CNC machine tool, modal analysis, failure mode, reliability, key meta-action unit.
The current decomposition methods are not suitable for electromechanical product; the smallest decomposition units obtained by these methods are static parts or components, which cannot reflect the characteristics of electromechanical product that ‘action determines motion, and motion determines function'. Meta‐action is the smallest action to realize the function of electromechanical product, and it is reasonable to regard meta‐action as the smallest decomposition unit of electromechanical product. Meta‐action unit is the smallest structural unit to ensure the normal operation of meta‐action, and it is also the smallest carrier of electromechanical product quality characteristics. Meta‐action unit modeling technology is the basis of other subsequent research, so it is necessary to study it. In this paper, detailed criteria for meta‐action decomposition and meta‐action unit separation are formulated, and the standardized structural model, symbolized conceptual model, and assembly model of meta‐action unit are also studied, which provide a basis for mechanical, kinematic, and failure mechanism research of electromechanical product. A CNC (computer numerical control) machine tool made in China is taken as an example for meta‐motion decomposition and meta‐action unit modeling, and the results verify the applicability and correctness of the method proposed in this paper. The proposed method is also applicable to other types of electromechanical product.
Traditional machine tool reliability modeling methods take static parts as the research object, ignoring the characteristic that the function and performance of computer numerical control (CNC) machine tool are ultimately driven by its most basic actions, and the traditional reliability modeling methods do not fully consider the randomness of machine tool failure. In view of this, this paper gave an improved four‐parameter nonhomogeneous Poisson process reliability modeling method based on meta‐action (MA), and a comprehensive method of system reliability was presented. A CNC machine tool made in China was taken as an example, and the reliability model of MA and the whole machine were built by the proposed method. The proposed method was compared with other methods by Akaike information criterion and Bayesian information criterion, and the method proposed in this paper was proved to be better. The reliability indexes of the system were simulated, and then the general trend of system reliability indexes was obtained by least squares support vector machines. The results verify the applicability and validity of this method, and lay a foundation for further research on the machine tool reliability based on metamotion.
The impact resistance of fiber-reinforced polymer composites is a critical concern for structure design in aerospace applications. In this work, experiments were conducted to evaluate the impact performance of four types of composite panels, using a gas-gun test system. Computational efficient finite element models were developed to model the high-speed ballistic impact behavior of laminate and textile composites. The models were first validated by comparing the critical impact threshold and the failure patterns against experimental results. The damage progression and energy evolution behavior were combined to analyze the impact failure process of the composite panels. Numerical parametric studies were designed to investigate the sensitivity of impact resistance against impact attitude, including impact deflection angles and projectile deflection angles, which provide a comprehensive understanding of the damage tolerance of the composite panels. The numerical results elaborate the different impact resistances for laminate and textile composites and their different sensitivities to deflection angles.
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